1. Field of the Invention
This invention relates generally to semiconductor technology, and more particularly, to maintaining substantially constant temperature of a semiconductor device under test.
2. Discussion of the Related Art
Semiconductor devices typically undergo a variety of electrical test procedures, including short-circuit tests, burn-in tests, and device functional tests to insure their proper operation. During for example functional testing, it is important that the temperature of the device under test be held at a chosen, substantially constant value. However, during such functional testing, the power level of the device may vary greatly, causing the temperature of the device to fluctuate. The most important parameter is junction temperature, or the temperature of active regions in the device (there may be some temperature non-uniformity within the device). In dealing with this problem, it is well known to provide a thermal head 10 a surface 11 which may be brought into contact with the lid 12 of a device 14 under test, for example, a flip-chip mounted on a printed circuit board 16 (FIG. 1), or in the case of an unlidded device, in contact with the device 14 itself (FIG. 2). The thermal head 10 typically includes an electric heating element 18 along surface 11 the output of which can be increased and decreased by respectively increasing and decreasing electrical current flow therethrough, and a passage 20 through which coolant 22, for example, water, may flow. By changing electrical current flow and/or providing or cutting off coolant flow, the temperature of the thermal head 10, and thus the temperature of the device under test 14 adjacent thereto, can be adjusted or varied. As the temperature of the device under test 14 varies due to changes in power level thereof as described above, the temperature of the thermal head 10 is caused to change to compensate for the changing temperature of the device 14, in order to attempt to maintain the device under test 14 at a constant, chosen temperature.
One approach in attempting to maintain the device under test 14 at a substantially constant temperature is to compare the temperature of the device under test 14 with a desired temperature as the temperature of the device under test 14 varies due to fluctuation of power level thereof. A PID (Proportional Integral Derivative) controller is used to sense that difference and vary the temperature of the thermal head 10 in order to bring the temperature of the device under test 14 back to the chosen value. However, this approach requires an accurate measurement of the temperature of the device under test 14, which cannot realistically be achieved with a lidded device if a temperature sensor is not incorporated in the device, and is also difficult even with an unlidded device. Additionally, in the case of a lidded device, because of the thermal capacitance of the lid, a substantial delay occurs in change of temperature of the device under test through change in the temperature of the thermal head. Thus, this approach has not proven entirely satisfactory.
Another approach, currently practiced by Schlumberger, Ltd. for unlidded devices uses an algorithm as follows:
Tc=Tdxe2x88x92Kxcex8Pd
where:
Td=temperature of device under test;
Tc=temperature of thermal head
Pd=power dissipated by device under test;
Kxcex8=thermal stack up coefficient of device (overall thermal resistance between the die and the thermal head).
In this approach, the device under test temperature Td is chosen and thermal head temperature Tc is set in accordance with this formula. The power dissipated by the device under test 14 is monitored. Through use of this formula, the temperature of the thermal head 10 can be varied in an attempt to hold the device under test 14 at a substantially constant temperature. However, it has been found that while ideally Kxcex8is a constant, this has proven not to be the case, that is Kxcex8may vary from one test run to another, causing inaccuracies in the attempt to hold the device under test 14 at a substantially constant temperature. Additionally, for functioning of this system, substantial, rapid swings in thermal head temperature are required, in turn requiring expensive and complicated hardware.
Therefore, what is needed is an approach in maintaining a device under test at a substantially constant temperature which overcomes the problems set forth above, meanwhile being simple, inexpensive and effective, and is equally effective in the case of lidded and unlidded devices.
In maintaining a device under test at a generally constant temperature, the temperature change of the device under test is characterized as the device under test undergoes changes in power level in response to an electrical testing sequence. Additionally, the temperature change of the device under test is characterized in response to changes in power level of a thermal head. Using this information, power levels at the thermal head are selected for use during the electrical testing sequence, based at least in part on the characterization of the temperature change of the device under test in response to the electrical testing sequence, so that the device under test remains at a substantially constant temperature during the electrical testing sequence.
The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there is shown and described an embodiment of this invention simply by way of the illustration of the best mode to carry out the invention. As will be realized, the invention is capable of other embodiments and its several details are capable of modifications and various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive.